The evolution towards higher power microprocessor and memory semiconductors has driven interest in highly conductive metal thermal interface materials (MTI) to provide the thermal connection between chip and heat sink.
Metal interfaces perform well as direct interfaces (between a semiconductor processor and heat-sink), or as indirect interfaces (between a semiconductor processor and a lid, or heat-spreader cap or plate, or between such a lid or cap or plate and a heat-sink). For good and reliable performance however, patterned metal thermal interfaces must be kept under compression, with a compressive pressure of the order of 100 psi (pounds per square inch). Presently, many high performance computer processor packages readily provide this level of compressive load, often in the range of 100 to 300 psi, and in some cases, as high as 1,000 psi.
The benefit of a high compressive load appears evident during short term testing of the PMTI: the larger the compressive load, the better the thermal conduction through the thermal interface. Stress testing of the computer package, however, brings out a weakness of the PMTI in the form of a degradation that can occur at a later date in the product life. Specifically, it occurs under intense thermal cycling, where the temperature is varied between room temperature and a higher operating temperature over many cycles that simulate many power-on and power-off operations of the computer.
PMTI is still a very new technology that is just being introduced into the computer industry. Its high thermal conductivity, ease of manufacturing and rework, make it very valuable as a key component of the cooling package of high power processor chips. It is also essential that it performs reliably over the life-time of the computer.
We have identified a potential failure mechanism that occurs after extended operation, and particularly as the results of temperature transients, that occur for example during power on or off operations. When the temperature changes, materials expand or contract due to thermal expansion. A copper heat-sink will expand by several tens of microns during a power-on operation. By contrast, a silicon chip will only expand by a few microns. The compliance of the malleable metal that comprises the PMTI is key to maintaining a good and reliable thermal bond between a copper heat-sink and a silicon chip, despite this non-uniform motion.
Over many such differential motion cycles however, some of the PMTI material is slowly squeezed out of the interfacial narrow volume between the heat-sink and chip. The squeezing out occurs as an inchworm-like motion under repeated temperature cycles. It occurs in a non-uniform manner: the motion is slow at the center of the chip, but is larger near the perimeter of the chip. Consequently, the interface first degrades at the periphery of the chip, as the result of a slight thinning of the PMTI material at these locations. Over time however, the PMTI can be squeezed out to such a level that the thermal bond substantially degrades over a large portion of the chip area.
Several solutions have been explored, which have yielded various levels of success. One of these, optimization of the compressive load, reduces the compressive load and slows down the squeeze-out motion of the PMTI. However, the overall performance and reliability also is diminished.
Another solution is using stiffer PMTI materials, like Tin or an Indium-tin alloy. Here the squeeze out motion is reduced, but so is the overall thermal bond, because of the more limited compressibility of the material.
Patterning the interfacial surfaces (heat-sink or chip): patterning the heat-sink can help anchor the PMTI material and slow the inchworm motion. However, the main effect of the patterned surface is to allow a more uniform bond between the PMTI and the surface, by providing many delocalized points of contact between the malleable metal and the surface. In another application, the patterned structure actually facilitates the squeezing out of the material, so as to minimize its thickness. This is generally desirable since the thermal bond often improves with decreasing interface thickness. However, beyond a minimum thickness level, the thermal bond then degrades, especially for the case of a PMTI.
Therefore, a need still exists for a solution to the above shortcomings.